Products, especially foods, are often packaged in film for sale to consumers. The film typically is wrapped around the product, bringing the edges of the film together forming a tube. The edges are sealed between rollers by pressure and heat, forming a lap seal. The ends of the tube are also sealed with pressure and heat, forming a cross seal. Such packaging and sealing techniques are used, for example, in modified atmosphere packaging, or MAP, also known as controlled atmosphere packaging, or CAP. In MAP, a product is wrapped in a film that is impervious to certain gasses, such as NO.sub.2 or CO.sub.2. A hermetic lap seal is formed. One end of the tube is hermetically cross sealed. The selected gas is then injected into the tube, blowing out the air. The selected gas inhibits the growth of bacteria and thus increases shelf life.
In all film packaging, whether MAP or otherwise, the quality of the seal is affected by the temperature of the film. The film must be at a sufficient temperature that it bonds when it is pressed together.
The width desired for the seal is affected by the product itself. A wider seal reduces leakage and increases package strength. A thinner seal reduces film used, makes a package easier to open, and gives the package a cleaner, tighter look which is important in a point of sale display. A wider seal is achieved by increasing the temperature of the film when it is pressed together.
Merely increasing the temperature of the heated rollers, whether to increase the quality or width of the seal, may result in burning the film. It is therefore desirable to increase the temperature of the film at sealing without increasing the temperature of the rollers.
It has been found that preheating the film can improve the seal by assuring that the film is at a certain temperature such that a seal of a desired quality will form. Further, heat seals are made wider when the film is preheated for a time before sealing. The seal can be of higher quality and wider by increasing the amount of preheating. Prior art sealing machines have not had an effective or convenient way of preheating the film. Further, there has not been a way of adjusting the amount of preheating to affect the width or quality of the seal.
U.S. Ser. No. 07/725,009 discloses a method and apparatus for sealing film by passing the film between sealing rollers, at least one of which is heated. To improve the seal, the film is preheated by laying it against the heated sealing roller before pressing the film between both sealing rollers. Since the film is preheated, a seal of higher quality and desired width results.